The present invention relates to power integrated circuits (ICs); more particularly, to an IC with a high voltage output device.
A common type of integrated circuit device is a metal-oxide-semiconductor field effect transistor (MOSFET). A MOSFET is a field-effect device that includes a source region, a drain region, a channel region extending between the source and drain regions, and a gate provided over the channel region. The gate includes a conductive gate structure disposed over and insulated from the channel region by a thin oxide layer.
Large, lateral MOSFET devices are widely used in power integrated circuits that operate at high voltages. Power ICs typically contain one or more large high-voltage output transistors that control the current flow to one or more external loads. In certain applications the high-voltage output transistor operates at a high switching speed. For instance, in a switch-mode power supply integrated circuit one output transistor controls the current through the primary winding of a transformer, and thereby controls the power delivered by the power supply. An example of a power supply circuit in which a switching transistor is coupled to the primary winding of a transformer is disclosed in U.S. Pat. No. 4,999,759. A power IC with an integrated circuit driver is described in U.S. Pat. No. 6,225,664 B1.
In a power IC, the large output transistor is typically designed as an array of elongated segments comprising the source and drain regions of the MOSFET. To increase the current handling capability of the output transistor, more segments are simply added to the device. In certain designs, the source and drain regions of the output transistor are arranged in an interdigitated manner in which the segments may include one or more pairs of source/drain xe2x80x9cfingersxe2x80x9d. By way of background, U.S. Pat. No. 5,258,636 describes a high-voltage transistor having interdigitated source drain regions.
Power ICs also generally include a control circuit that controls the output transistor. The control circuit generates a signal that is applied to the gate of the output transistor to turn it on and off. This signal is produced by a gate driver, which typically comprises a relatively large PMOS transistor for turning on, and a relatively large NMOS transistor for turning off the output transistor. The speed at which the output transistor is turned on and off is a factor in determining the switching power loss of the output device. This becomes an increasingly important power loss component of the power IC as the switching frequency is increased. The distributed gate capacitance of the output transistor and the effective resistance between this capacitance and the turn-on voltage (typically VDD) and the turn-off voltage (typically VSS) largely determine the switching speed of the power device.
In prior art devices, the gate driver is located in the control circuit section on the IC chip, and a long bus is used to connect the output of the gate driver to the gate electrodes of each segment of the output transistor. Such an arrangement is shown in FIG. 1.
FIG. 1 is a plan view of a typical prior art power IC 10 that includes agate driver 12 physically located in the same section of the chip as control circuit 11. A bus 13 connects the output of gate driver 12 to the gate electrodes of output transistor 14. Although not depicted in detail in the example of FIG. 1, output transistor 14 has four segments, with each segment including multiple pairs of elongated source/drain fingers. Each segment of output transistor 14 has an associated drain pad 15 and source pad 16.
One of the shortcomings of the prior art power IC of FIG. 1 is that the gate driver is required to be large enough to provide fast switching of the largest output device connected to a given control circuit. This requirement means that the gate driver is generally oversized for the products that comprise a smaller output transistor connected to the same control circuit, and thus wastes silicon area. Another drawback is that the bus that connects the gate driver to each of the segments of the output transistor becomes longer, and therefore more resistive, for larger output transistors. Higher resistance at this node in the circuit has an adverse affect on switching power loss of the power IC.